In recent years, along with a tendency to high brightness and whitening of a light emitting device having a light emitting element such as a light emitting diode, a light emitting device using a light emitting element has been used for a backlight of a mobile phone, a liquid crystal TV, etc. In such a light-emitting device, a substrate is required to have a high thermal conductivity, be able to quickly dissipate heat generated from the light-emitting element and in addition, have a high reflectance and excellent productivity.
Further, the substrate is required to have a designated strength in order to prevent breakage, fracture, etc. due to a stress exerted on the substrate at the time of mounting a light emitting element.
In order to satisfy such requirements, it has been studied to use a glass ceramic substrate as a substrate for a light emitting device. The glass ceramic substrate comprises a glass powder and a ceramic powder such as an alumina powder, and it has a large difference in refractive index between glass and ceramics and many interfaces between them, whereby the glass ceramic substrate has a higher reflectance than conventional ceramic substrates. However, the glass ceramic substrate is required to have a further high reflectance to be used as a substrate for mounting a light emitting element.
Further, for reducing unevenness in various properties such as reflectance and strength, the glass ceramic substrate is also required to have a property to suppress shrinkage at the time of firing, namely firing shrinkage.
In order to increase the reflectance of a glass ceramic substrate, a method of incorporating ceramic particles having a refractive index higher than that of alumina particles, namely high refractive index particles, has been studied. However, one containing high refractive index particles tends to have a low sintering property as compared with one containing fillers of e.g. alumina or silica (SiO2). Accordingly, the content of such fillers cannot be made high, or the composition of glass is significantly restricted. Thus, the degree of freedom of the design is decreased. Accordingly, in order to make it possible to select the glass composition from a wide range, it is desired to increase the reflectance and reduce firing shrinkage by using alumina particles having a good sintering property with glass.
As a method for suppressing firing shrinkage of glass ceramic substrates, a method has been known wherein as ceramic particles, flat particles having an aspect ratio of 5 are aligned to the designated direction (for example, Patent Document 1). Further, as a method for improving the strength, a method has been known wherein ceramic particles having an aspect ratio of at least 4 and at most 10 are dispersed and incorporated (for example, Patent Document 2).
However, by the methods described in Patent Documents 1 and 2, it was not possible in either case to obtain a sufficiently high reflectance as a substrate for mounting a light emitting element.
Patent Document 3 discloses a method for improving the reflectance by a glass ceramic substrate comprising glass and ceramic particles. In Patent Document 3, a light reflection body with a high reflectance is proposed wherein in the cross section viewed from the top of the glass ceramic substrate, the occupation area of the group of particles of ceramic particles having a particle size of from 0.3 to 1.0 μm is made to be from 10 to 70%.
In the light reflection body disclosed in Patent Document 3, fine ceramic particles having a particle size of from 0.3 to 1.0 μm are incorporated in a designated amount in glass to obtain a high reflectance. However, if such fine ceramic particles are incorporated in a large amount in glass, the sintering property of a glass ceramic body tends to be low, the strength of the substrate tends to be low, and voids are likely to be formed at the surface of the substrate.
If such a glass ceramic body is used as a substrate for mounting a light emitting element, breakage due to stress exerted on the substrate at the time of mounting a light emitting element and fracture of an individual piece after cutting tend to result, and thereby the yield may be low.
Further, in the case of the glass ceramic body of Patent Document 3, the crystallinity is at least 50%, and thereby, at the time of firing, the flowability and the sintering property tend to be low due to the crystallization of glass components, and voids are likely to be formed in the substrate and at the surface of the substrate.
If voids are formed in the substrate, although the reflectance becomes slightly high, the strength of the substrate deteriorates, and a plating solution used for metal plating treatment tends to infiltrate into the inside of the substrate from the voids. Thus, after mounting a light emitting element, defects such as connection defects tend to result, and the reliability may deteriorate.